1. Field of the Invention
The present invention relates generally to a method and apparatus for multiplexing frequency hopping in a communication system, and in particular, to a frequency hopping multiplexing method and apparatus for efficiently indicating time division multiplexing for wide-band frequency hopping and sub-band frequency hopping in a wireless communication system using Frequency Division Multiple Access.
2. Description of the Related Art
Recently, in mobile communication systems, extensive search is being conducted into Orthogonal Frequency Division Multiplexing (OFDM) as a scheme for high-speed data transmission in wire/wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is one type of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbol streams and modulates each of the parallel symbol streams with a plurality of orthogonal sub-carriers, i.e., a plurality of orthogonal sub-carrier channels, before transmission.
A system that distinguishes several users using the multiple sub-carriers while adopting OFDM as its basic transmission scheme, in other words, a system that supports several users, with a scheme of allocating different sub-carriers to different users, is an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
FIG. 1 is a diagram illustrating an example where a terminal transmits data over arbitrary allocated resources in a general OFDMA system. FIG. 1 is composed of one or multiple sub-carriers in the frequency domain, and of one or multiple OFDM symbols in the time domain. In FIG. 1, reference numeral 101 indicates resources that a terminal 1 MS1 uses to transmit data, and reference numeral 103 indicates resources that a terminal 2 MS2 uses to transmit data. The term ‘resource’ as used herein refers to resources in the time-frequency domain, and indicates OFDMA symbols in the time domain and sub-carriers in the frequency domain.
Referring to FIG. 1, the resources 101 and 103 that the terminal 1 and the terminal 2 use to transmit data, consecutively occupy particular frequency bands with the passage of time. This resource allocation scheme or data transmission scheme is widely used for the case where there is an intention to select a frequency band having a good channel state and allocate the selected frequency band to each terminal, thereby maximizing the system performance with the limited system resources.
For example, in FIG. 1, for a wireless channel that the terminal 1 experiences, the parts indicated by reference numeral 101 are relatively superior to other frequency bands in the frequency domain, and for a wireless channel that the terminal 2 experiences, the parts indicated by reference numeral 103 are relatively superior to other frequency bands in the frequency domain. A scheme of selectively allocating resources by selecting frequency bands having a superior channel response in the frequency domain is generally referred to as ‘frequency selective resource allocation’ or ‘frequency selective scheduling’.
Although the foregoing description has been given with reference to the uplink (or reverse link), i.e., data transmission from a terminal to a base station, for convenience, the same can be applied even to the downlink (or forward link), i.e., data transmission from a base station to a terminal. In the case of the downlink, in FIG. 1, the parts indicated by reference numerals 101 and 103 indicate resources that the base station uses to transmit data to the terminal 1 and resources that the base station uses to transmit data to the terminal 2, respectively.
The frequency selective scheduling illustrated in FIG. 1 is not always available. For example, for a terminal moving at a high speed, since its channel state varies quickly, the frequency selective scheduling is unavailable for the terminal for the following reasons. When a base station scheduler allocates resources to a particular terminal by selecting a frequency band having a superior channel state, the terminal receives resource allocation information from the base station and actually transmits data over the allocated resources. However, since the channel environment has already changed greatly at an actual data transmission time, the selected frequency band is not guaranteed to still have a superior channel state. There is a frequency hopping scheme as an available scheme for this case. It should be noted that the use of the frequency hopping is not limited only to the case where the frequency selective scheduling is unavailable.
FIG. 2 is a diagram illustrating an example where a frequency hopping scheme is used in a general OFDMA system. Referring to FIG. 2, it can be noted that resources that one terminal uses to transmit data continuously change (or hop) with the passage of time. This frequency hopping scheme contributes to randomizing the interference that data transmission suffers, and the channel quality.
In the general wireless communication system, a Hybrid Automatic Repeat reQuest (HARQ) technology is one of the major technologies used for increasing the reliability of data transmission and the data throughput. The term ‘HARQ’ refers to a combined technology of an Automatic Repeat reQuest (ARQ) technology and a Forward Error Correction (FEC) technology. In the ARQ technology widely used in the wire/wireless data communication system, a transmitter assigns sequence numbers to data packets according to a predefined scheme before transmission, and a receiver sends to the transmitter a retransmission request for a data packet(s) with a missing sequence number among the received data packets, thereby achieving reliable data transmission. The term ‘FEC’ refers to a technology for adding redundant bits to transmission data according to a predetermined rule before transmission, like convolutional coding and/or turbo coding, thereby coping with noise generated in a data transmission/reception process and/or errors occurring in the fading environment and the like, and thus demodulating the originally transmitted data.
In a system using HARQ proposed by combining the two technologies ARQ and FEC, a data receiver determines if there are any errors in the received data by performing a Cyclic Redundancy Check (CRC) check on the data decoded by way of a predetermined inverse FEC process. If there are no errors as a result of the CRC check, the receiver feeds back an Acknowledgement (ACK) to a transmitter so that the transmitter may transmit the next data packet, and if it is determined that there is an error in the received data, the receiver feeds back a Non-Acknowledgement (NACK) to the transmitter so that the transmitter may retransmit the previously transmitted packet. In the retransmission process, the receiver combines the retransmitted packet with the previously transmitted packet, thereby obtaining energy and coding gain. Therefore, with the use of HARQ, the communication system can obtain performance remarkably improved from that of the communication system using the conventional ARQ with no combining process.
The communication system employing HARQ basically employs the frequency hopping scheme in order to obtain a diversity effect in the forward and reverse transmissions. To obtain the diversity effect, the communication system uses a Distributed Resource Channel (DRCH) in the forward link and a wide-band frequency hopping scheme in the reverse link. To maximize the diversity effect in the reverse link, a multiplexing ratio of slots using the wide-band frequency hopping scheme to slots using a sub-band frequency hopping scheme is very important. However, in the current OFDMA system, no scheme has been proposed for efficiently multiplexing wide-band frequency hopping and sub-band frequency hopping so as to maximize the diversity effect, without additional overhead. Therefore, there is a need for a scheme for efficiently managing frequency hopping multiplexing in the forward and reverse links in a communication system.